The invention relates to electrical circuit arrangements and more specifically to circuit arrangements for detecting the operation and identity of one of a plurality of switching elements. By way of example, the circuit arrangement to be described may be used in connection with a security installation involving a plurality of sensors each incorporating a respective switching element which is operated when the sensor detects an alarm state, so as to enable the operation and the identity of an operating sensor to be determined.
In security installations used for protecting an area under surveillance from intrusion, it is known to position a number of sensors at different locations in and around the area under surveillance so that the presence of an intruder is indicated at a central monitoring point to which the sensors are connected. In practice, it is desirable to know at the central monitoring point not only that a sensor has operated but also the identity of the sensor (that is, the position within the area under surveillance where the intrusion has taken place). If each sensor is individually wired back to the central monitoring point, then this facility may be achieved, but such an arrangement is complex and time-consuming to install. Circuit arrangements intended to deal with this problem will be described below.